Apparatus and method for detecting an endpoint in a vapor phase etch

ABSTRACT

Processes for the removal of a layer or region from a workpiece material by contact with a process gas in the manufacture of a microstructure are enhanced by the ability to accurately determine the endpoint of the removal step. A vapor phase etchant is used to remove a material that has been deposited on a substrate, with or without other deposited structure thereon. By creating an impedance at the exit of an etching chamber (or downstream thereof), as the vapor phase etchant passes from the etching chamber, a gaseous product of the etching reaction is monitored; and the endpoint of the removal process can be determined. The vapor phase etching process can be flow through, a combination of flow through and pulse, or recirculated back to the etching chamber

BACKGROUND OF THE INVENTION

This U.S. patent application is a divisional patent application ofco-pending U.S. patent application Ser. No. 10/269,149 to Patel et alfiled Oct. 10, 2002, which is a continuation in part of U.S. patentapplication Ser. No. 09/954,864 to Patel et al., filed Sep. 17, 2001.This application is related to U.S. patent application Ser. No.09/427,841, to Patel et al., filed Oct. 26, 1999, and U.S. patentapplication Ser. No. 09/649,599 to Patel et al, filed Aug. 28, 2000, thesubject matter of each being incorporated herein by reference.

End point detection in plasma etching reactions is known in the art.However, end point detection by monitoring gases with a gas analyzer ina non-plasma system has not been available till now, particularly in aflow-through or recirculation etch system. Though other types of endpoint detection methods have been used in etch systems (opticalmonitoring, electrical monitoring, etc.), such methods can be difficultto set up and inaccurate.

The present invention is in the area of the manufacture of MEMS(microelectromechanical systems) as well as semiconductor devices, orany other devices that require removal of a material in accordance withthe present invention. In particular, this invention addresses gas-phaseetching procedures, with particular emphasis on detection of theendpoint in an etching process. The invention is also directed toapparatus useful for etching and detecting the endpoint of the etchingreaction. “MEMS”, “microelectromechanical” and “micromechanical” areused interchangeably throughout this application and each may or may nothave an electrical component in addition to the microstructurecomponent. The end point detected can be a point in an etch processwhere all of the material that is capable of reacting with the etchantgas has been removed and there is no more of the material remaining onthe substrate or exposed to the etchant gas.

The use of etchants for removing sacrificial layers or regions in amultilayer structure without removal of an adjacent layer or region is acommon step in the manufacture of semiconductor and MEMS devices. TheMEMS devices of the present invention can be devices for inertialmeasurement, pressure sensing, thermal measurement, micro-fluidics,optics, and radio-frequency communications, with specific examplesincluding optical switches, micromirror arrays for projection displays,accelerometers, variable capacitors and DC or RF switches. If asemiconductor device is etched, it can be any device that is made of orhas thereon a material that is to be removed with a gas phase chemicaletchant.

The success of an etch step in the manufacture of microstructures isimproved not only due to the selectivity of the etchant, but also due tothe ability to accurately determine the endpoint of the etching process.Isotropic etching is of particular interest in processes where thepurpose of the etch is to remove a sacrificial layer that is interveningbetween functional layers or between a functional layer and a substrate.Gas phase etchants, particularly in the absence of plasma, are desirablefor isotropically removing a sacrificial layer.

Of potential relevance to certain embodiments of this invention is theprior art relating to particular etchant gases. Prominent among theetchants that are used for the removal of sacrificial layers or regionsin both isotropic and anisotropic etching procedures are noble gasfluorides and halogen fluorides. These materials, used in the gas phase,selectively etch silicon relative to other materials such assilicon-containing compounds, non-silicon elements, and compounds ofnon-silicon elements. Descriptions of how these materials are used inetching procedures appear in co-pending U.S. patent application Ser.Nos. 09/427,841 and 09/649,569 to Patel et al. and in portions of thepresent specification that follow.

The method of the present invention is useful for detecting an endpointin methods for producing deflectable MEMS elements (deflectable byelectrostatic or other means) which, if coated (before or after gasphase processing) with a reflective layer, can act as an actuatablemicromirror. Arrays of such micromirrors can be provided for direct viewor projection display systems (e.g. projection television or computermonitors). If the micromirrors are provided alone or in an array and ofa size of preferably 100 micrometers or more (preferably 500 micrometersor more), the mirror can be useful for steering light beams, such as inan optical switch. The present invention is also adaptable to detectingan endpoint in methods for etching microfabricated devices other thanMEMS devices (e.g. semiconductor based devices, carbon nanotubes onglass, etc.)

SUMMARY OF THE INVENTION

The present invention provides improvements in the apparatus and methodsused for the etching of layers or areas, and in particular, fordetermining an end of the etching reaction. In one embodiment of theinvention, a method for etching a sample comprises: providing a sampleto be etched in a chamber; providing a vapor phase etchant to thechamber to etch the sample, the vapor phase etchant capable of etchingthe sample in a non-energized state; monitoring the gas from the etchingchamber; and determining the end point of the etch based on themonitoring of the gas from the etching chamber.

Another example of the invention is a method for etching a sample,comprising: providing a sample to be etched to an etching chamber;passing a gas phase etchant through the etching chamber; impeding thegas flow out of the etching chamber, wherein the impedance is less thaninfinity but greater than 0; analyzing the gas from the etching chamberand determining an end of the etch.

A further aspect of the invention is an etching method, that comprisesetching a material from a sample with a gas phase etchant; monitoringone or more gas components from the etching reaction substantially inthe absence of plasma and determining the endpoint of the etchingreaction based on the monitoring of the one or more gas components.

Another embodiment of the invention is a method for etching a material,comprising: performing an etch on a material on a substrate by providingan etchant so as to chemically but not physically etch the material onthe substrate; monitoring an etch product of the material being etched;and determining an endpoint of the etch of the material based on themonitoring of the etch product.

Still another example of the invention is a method, comprising: a)providing a sample to be etched in a chamber; b) providing an etchant tothe chamber, capable of etching the sample; c) providing no orsubstantially no impedance to gas exiting the etching chamber; d)monitoring a partial pressure of an etch product; repeating steps a) tod) except providing an increased impedance each time steps a) to d) arerepeated, until an impedance is reached that allows for determining anendpoint based on monitoring the partial pressure of the etch product.

Another part of the invention is an apparatus. The apparatus comprisesan etching chamber; a source of a vapor phase spontaneous chemicaletchant; a gas flow line for recirculating the etchant; a gas analyzerconnected to the etching chamber or to the gas flow line downstream ofthe etching chamber, though preferably upstream of any impedance in thegas out flow line from the etching chamber.

Another example of the apparatus of the invention comprises: an etchingchamber; a source of etchant capable of being in fluid communicationwith an entrance aperture in the etching chamber; an exit flow lineconnected to an exit aperture in the etching chamber; and an impedancevalve within the exit flow line for providing an impedance to the gasflow out of the etching chamber.

Yet another apparatus in accordance with the present invention is anapparatus comprising: an etching chamber; a holder for holding a sampleto be etched; a source of gas phase etchant for supplying a gas phaseetchant to the etching chamber, wherein the gas phase etchant is afluoride compound capable of etching a sample in a non-energized state;and a gas analyzer for analyzing gas components from the etching of thesample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of a system for etching andstopping the etch in accordance with the present invention;

FIG. 2 is a diagram of a second example of a system for etching andstopping the etch in accordance with the present invention;

FIG. 3A is a side elevation view of one example of a reciprocating pumpfor use in one embodiment of the invention;

FIG. 3B is a pump flow diagram of the reciprocating pump of FIG. 3Atogether with associated flow lines and shutoff valves;

FIG. 4 is a process flow diagram for the apparatus of FIG. 2;

FIG. 5 is a graph of the partial pressure (ion current in a residual gasanalyzer) of different etching products vs. time in the invention;

FIG. 6A is a graph of the partial pressure of SiF3 vs. time;

FIG. 6B is a graph of the data of FIG. 6A back averaged over 40 previousdata points;

FIG. 7A is a graph of the derivative taken from the data of FIG. 6B;

FIG. 7B is a graph of the data of FIG. 7A back averaged over 40 previousdata points; and

FIG. 8 is a graph of the partial pressure (ion current in a residual gasanalyzer) of different etching products vs. time in a prior art methodand apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While this invention is susceptible to a variety of constructions,arrangements, flow schemes, and embodiments in general, the novelfeatures that characterize the invention are best understood by firstreviewing a typical process flow arrangement in which the variousaspects of this invention might be used.

As can be seen in FIG. 1, an apparatus is provided for etching a samplethat includes a source chamber 11 containing a source of chemicaletchant, maintained at a particular temperature and pressure formaintaining the etchant source in a solid or liquid state (e.g. solidstate for XeF2 crystals, liquid state for BrF3, etc.). An expansionchamber 12 is in fluid communication with source chamber 11 and has anysuitable size (e.g. a volumetric capacity of 29 cubic inches (0.46liter)) to receive etchant gas from the source chamber 11, with ashutoff valve 13 joining these two chambers. An etch chamber 15 isprovided in fluid communication with expansion chamber 12 and has anysuitable size (e.g. volumetric capacity of 12 cubic inches (0.18 liter))to contain the sample microstructure to be etched. It is preferred thatthe etch chamber be smaller than the expansion chamber. The etch chamber15 is connected to the expansion chamber 12 via a shutoff valve 85. Alsoincluded in the apparatus is a first gas source 19 in fluidcommunication with the expansion chamber 12 via a further shutoff valve21, a second gas source 20 in fluid communication with the expansionchamber through a separate shutoff valve 22, a vacuum pump 23 andassociated shutoff valves 24, 25 to control the evacuation of thechambers.

Also shown in FIG. 1 are a third gas source 29 serving as a pump ballastwith an associated shutoff valve 30 to prevent backstreaming from thepump 23, and needle valves 32, 33, 31 to set the gas flow rates throughthe various lines and to permit fine adjustments to the pressures in thechambers. Also shown, as will be discussed in more depth below, are gasanalyzer 1 and valves 3 and 5 on opposite sides of the analyzer. Theexpansion chamber 12 and the etch chamber 15 can both be maintained at aparticular temperature, while different gases are placed in the firstand second gas sources for the various etching processes. It should benoted that a single gas source could be used in place of gas sources 19and 20.

The general procedure followed in these experiments began with theevacuation of both the expansion chamber 12 and the etch chamber 15,followed by venting both chambers to atmospheric pressure with gas fromthe first gas source 19 by opening the two shutoff valves 21, 85,between this gas source and the two chambers. The sample was then placedin the etch chamber 15 (with the shutoff valves 21, 85 open during thesample insertion) which was then sealed, and both the expansion chamber12 and the etch chamber 15 were evacuated. All valves were then closed.

The connecting valve 85 between the expansion chamber 12 and the etchchamber 15 was opened, and the shutoff valve 21 at the outlet of thefirst gas source 19 was opened briefly to allow the gas from the firstgas source to enter the expansion and etch chambers. The shutoff valve21 is then closed. The connecting valve 85 is then closed, and theexpansion chamber 12 is evacuated and isolated. The supply valve 13 fromthe etchant source chamber 11 is then opened to allow etchant gas toenter the expansion chamber (due to the higher temperature of theexpansion chamber). The supply valve 13 is then closed, outlet valve 25is opened, and the needle valve 33 is opened slightly to lower theetchant pressure in the expansion. Both the outlet valve 25 and theneedle valve 33 are then closed. The shutoff valve 22 at the second gassource 20 is then opened and with the assistance of the needle valve 32,gas from the second gas source is bled into the expansion. At this pointthe expansion chamber 12 contains the etchant gas plus gas from thesecond gas source 20, while the etch chamber 15 contains gas from thefirst gas source.

With pump 23 on, the connecting valve 85 between the expansion chamber12 and the etch chamber 15 is then opened, and valves 3 and 5 are openedon both sides of gas analyzer 1, to allow the gas mixture from theexpansion chamber to enter the etch chamber and flow through the etchchamber and gas analyzer, thereby beginning the etch process. As will bediscussed further below, the etch process is continued until an endpoint is detected via the gas analyzer.

Many alternatives to the process scheme described above can be used.Additional gas sources and chambers, for example, can be utilized. Forexample, depending upon the diluent(s) used (gas sources 19 and 20), aplurality of diluent sources (N2, Ar, He, etc.) can be connected to theexpansion chamber and/or to the recirculation loop for bleeding thesystem after an etch. The air distribution system within the etchingchamber can also be varied, for example by including U-shaped orcone-shaped baffles, or by using additional perforated plates and/orbaffles.

A specific alternative to the embodiment of FIG. 1 is illustrated inFIG. 2. FIG. 2 represents such a process flow arrangement in which theprocess is an etching process having a detectable end point. The etchantgas originates in a source chamber 11. An example of an etchant gas thatis conveniently evaporated from a liquid is bromine trifluoride, whereasan example of an etchant gas that is conveniently sublimated from solidcrystals is xenon difluoride. Effective results can be achieved bymaintaining the crystals under 40 degrees C. (e.g. at a temperature of28.5° C.). (Xenon difluoride is only one of several etchant gases thatcan be used. Examples of other gases are presented below.) Thesublimation pressure of xenon difluoride crystals at 28.5° C. is 5-11mbar (4-8 torr). An expansion chamber 12 receives xenon difluoride gasfrom the crystals in the source chamber(s) 11, and a shutoff valve 13 ispositioned between the source and expansion chambers. The sample 14 tobe etched is placed in an etch chamber 15 (which contains a baffle 16 aperforated plate 17), and a reciprocating pump 18 that is positionedbetween the expansion chamber 12 and the etch chamber 15. (Thereciprocating pump and its valves are shown in more detail in a FIGS. 3a and 3 b and described below.) Also illustrated in FIG. 2, and will bediscussed further below, is a gas analyzer 1 with valves 3 and 5 thatcontrol the flow of gas from the etching chamber through the gasanalyzer.

Also shown are two individual gas sources 19, 20 supplying the expansionchamber 12 through shutoff valves 21, 22, a vacuum pump 23 andassociated shutoff valves 24, 25, 26, 27, 28 to control the evacuationof the chambers, a third gas source 29 serving as a pump ballast with anassociated shutoff valve 30 to prevent backstreaming from the pump 23,and manually operated needle valves 31, 32, 33, 34, 35, 83 to set thegas flow rates through the various lines and to permit fine adjustmentsto the pressures in the chambers. When xenon difluoride is used, theexpansion chamber 12 and the etch chamber 15 are typically maintained ataround room temperature (e.g. 25.0° C.). However, the expansion chamberand etch chamber could also be heated (e.g. to between 25 and 40 degreesC.), though this would likely be performed in conjunction with directlycooling the sample being processed, as will be discussed below. Arecirculation line 36 permits gas to flow continuously through the etchchamber 15 in a circulation loop that communicates (via valves 26, 27,and 34, 35) with the expansion chamber 12 and reenters the etch chamber15 by way of the reciprocating pump 18. Valve 85 permits gas transferbetween expansion chamber 12 and etch chamber 15 via a portion of therecirculation line 36 without traversing recirculation pump 18. Valve 86in path 40 permits introduction of etchant gas into the expansionchamber 12 to replenish the etchant mixture during the etching process.The valves are preferably corrosive gas resistant bellows-sealed valves,preferably of aluminum or stainless steel with corrosive resistantO-rings for all seals (e.g. Kalrez™ or Chemraz™). The needle valves arealso preferably corrosion resistant, and preferably all stainless steel.A filter 39 could be placed in the recirculation line 36 to remove etchbyproducts from the recirculation flow (though preferably not theproduct(s) being monitored for end point detection), thereby reducingthe degree of dilution of the etchant gas in the flow. The filter canalso serve to reduce the volume of effluents from the process.

The etch chamber 15 can be of any shape or dimensions, but the mostfavorable results will be achieved when the internal dimensions andshape of the chamber are those that will promote even and steady flowwith no vortices or dead volumes in the chamber interior. A preferredconfiguration for the etch chamber is a circular or shallow cylindricalchamber, with a process gas inlet port at the center of the top of thechamber, plus a support in the center of the chamber near the bottom forthe sample, and an exit port in the bottom wall or in a side wall belowthe sample support. The baffle 16 is placed directly below the entryport. The distance from the port to the upper surface of the baffle isnot critical to this invention and may vary, although in preferredembodiments of the invention the distance is within the range of fromabout 0.1 cm to about 6.0 cm, and most preferably from about 0.5 cm toabout 3.0 cm. Although its shape is not shown in FIG. 2, the baffle ispreferably circular or otherwise shaped to deflect the gas streamradially over a 360° range. The perforated plate 17 is wider than thebaffle 16 and preferably transmits all gas flow towards the sample. Apreferred configuration for the perforated plate is one that matches thegeometry of the sample; thus, for a circular sample the perforated plateis preferably circular as well.

FIGS. 3 a and 3 b are diagrams of an example of a reciprocating pump 18that can be used in the practice of this invention. The design shown inthese diagrams can be varied in numerous ways, such as by increasing thenumber of chambers to three or more, or by arranging a series of suchpumps in parallel. The following discussion is directed to theparticular design shown in these diagrams.

The side elevation view of FIG. 3 a shows the pump housing 41, whichconsists of two stationary end walls 42, 43 joined by bellows-type sidewalls 44, 45. The bellows-type side walls 44, 45 are so-called becausethey are either pleated like an accordion or otherwise constructed topermit bellows-type expansion and contraction. The end walls 42, 43 andthe bellows-type side walls 44, 45 together fully enclose the interiorof the pump except for inlet/outlet ports on each side wall. Thearrangement of these ports is shown in the pump flow diagram of FIG. 3 b, the left side wall 42 having one inlet/outlet port 46, and the rightside wall 43 likewise having one inlet/outlet port 48. Remotelycontrolled shutoff valves 51, 52, 53, 54 are placed on the externallines leading to or from each inlet/outlet port. For fail-safeoperation, shutoff valves 51, 54 are normally open and shutoff valves52, 53 are normally closed.

The movable partition 60 shown in FIG. 3 a divides the pump interiorinto two chambers 61, 62, the partition and its connections to theremaining portions of the housing being fluid-impermeable so that thetwo chambers are completely separate with no fluid communication betweenthem. The partition 60 joins the bellows-type side walls 44, 45 andmoves in the two directions indicated by the two-headed arrow 63. Themovement is driven by a suitable drive mechanism (not shown) capable ofreciprocating movement. Many such drive mechanisms are known to thoseskilled in the art and can be used. In the view shown in FIG. 3 a,movement of the partition to the left causes the left chamber 61 tocontract and the right chamber 62 to expand. With the pump shutoffvalves appropriately positioned, i.e., valves 52 and 53 open and valves51 and 54 closed, the contracting left chamber 61 will discharge itscontents through its inlet/outlet port 46 while the expanding rightchamber 62 will draw gas in through its inlet/outlet port 48. Once thepartition 60 has reached the end of its leftward travel, it changesdirection and travels to the right and the shutoff valves are switchedappropriately, causing the expanded right chamber 62 to contract anddischarge its contents through its inlet/outlet port 48 while thecontracted left chamber 61 expands and draws fresh gas in through itsinlet/outlet port 46. In this manner, the pump as a whole produces a gasflow in a substantially continuous manner, the discharge comingalternately from the two chambers. A controller 64 governs the directionand range of motion, and the speed and cycle time of the partition 60,and coordinates the partition movement with the opening and closing ofthe shutoff valves 51, 52, 53, and 54. Conventional controller circuitryand components can be used.

The pump for recirculating the process gas as shown, and others withinthe scope of this invention, has no sliding or abrading parts orlubricant that come into contact with the process gas. Alternative pumpsthat meet this criteria are possible, including pumps with expandableballoon chambers, pumps with concentric pistons connected by an elasticsealing gasket, or peristaltic pumps. The pump materials, including thebellows-type walls, can thus be made of materials that are resistant orimpervious to corrosion from the etchant gas. One example of such amaterial, useful for operating temperatures below 50° C., is stainlesssteel. Others are aluminum, Inconel, and Monel. Still others will bereadily apparent to those experienced in handling these gases. While thecapacity and dimensions of the pump and its chambers may vary, apresently preferred embodiment is one in which the change in volume ofeach chamber upon the movement of the partition across its full range isapproximately from 0.05 to 4.2 L, though preferably from 0.1 to 1.5 L,with one example being 0.5 L. Larger chamber sizes (e.g. 5 to 20 L) arepossible, which, if combined with a slower pumping speed, can benefitfrom lower wear on the pump. At a partition speed of one cycle every twoseconds, the pump rate (for 0.5 L) will be 30 L/min. Differentcombinations of pump sizes and pump speeds are possible, though thepreferred pump volume per time is between 7 and 150 L/min, with apreferred range of from 30 to 90 L/min.

The pump described above can be lined with a suitable lining to furtherreduce particulate contamination of the process gas mixture duringetching. Pumps that are not of the bellows type can also be used. Thepreferred pumps are those that are resistant to corrosion by the processgas mixture and those that are designed to avoid introducing particulateor liquid material into the process gas mixture. Dry pumps, i.e., thosethat do not add exogenous purge or ballast gas into the process gasmixture, are preferred. Alternatively, the process gas could becirculated by temperature cycling (with large variations in the heatingand cooling of the recirculation path).

The following is a generalized description of the etching process andits chemical parameters in relation to FIG. 2. This description isincluded to show the context in which the features described above aremost likely to be used.

The apparatus and methods of this invention can be used in etchingprocesses that are known in the art and in the literature. Theseprocesses include the use of dry-etch gases in general, including C12,HBr, HF, CC12F2 and others. Preferred etchant gases, particularly foretching silicon, are gaseous halides (e.g. fluorides) such as noble gasfluorides, gaseous halogen fluorides, or combinations of gases withinthese groups (again, preferably without energizing the gas, other thanheating to cause vaporization or sublimation). The noble gases arehelium, neon, argon, krypton, xenon and radon, and among these thepreferred fluorides are fluorides of krypton and xenon, with xenonfluorides the most preferred. Common fluorides of these elements arekrypton difluoride, xenon difluoride, xenon tetrafluoride, and xenonhexafluoride. The most commonly used noble gas fluoride in silicon etchprocedures is xenon difluoride. Halogen fluorides include brominefluoride, bromine trifluoride, bromine pentafluoride, chlorine fluoride,chlorine trifluoride, chlorine pentafluoride, iodine pentafluoride andiodine heptafluoride. Preferred among these are bromine trifluoride,bromine trichloride, and iodine pentafluoride, with bromine trifluorideand chlorine trifluoride the more preferred. Combinations of brominetrifluoride and xenon difluoride are also of interest. The etch processis generally performed at a pressure below atmospheric. It is preferredthat the etchants described herein be used in the gaseous state (e.g.non-plasma) or otherwise in the absence of added energy (except heat toaid sublimation or vaporization of the starting etchant gas or liquid),and in the absence of electric fields, UV light or other electromagneticenergy, or other added fields or energy sources which would energize theetchant gas beyond it's normal energy as a gas at a particulartemperature.

The etch preferably utilizes an etchant gas capable of spontaneouschemical etching of the sacrificial material at room temperature,preferably isotropic etching that chemically (and not physically)removes the sacrificial material. In a preferred embodiment, the etchantis capable at room temperature of reacting with the sacrificial materialand where the reaction product(s) is a gaseous component that isreleased from the sacrificial material surface due to being in a gaseousstate. No UV or visible light or other electromagnetic radiation orelectric fields are needed, or any energy that would energize the gasmolecules to physically bombard and physically remove the sacrificialmaterial. Though the etch could be performed with the application ofheat or the presence of light from the room surrounding the etchchamber, the etchant is capable of spontaneously etching the sacrificialmaterial at room temperature without any applied heat, visible, UV orother electromagnetic radiation, ultrasonic energy, electric or magneticfields, etc. The etchant is preferably not broken down into atoms,radicals and/or ions by an rf glow discharge, the etchant is transportedby diffusion to the surface of the material being etched (in addition topumping—e.g. by recirculating the etchant repeatedly through the etchingchamber), the etchant is adsorbed on the surface, a chemical reactionoccurs between the etchant and the material being etched with theformation of a volatile product, and the product is desorbed from thesurface and diffuses into the bulk of the gas and eventually exits theetching chamber. The apparatus, therefore, can be without a source of RFor microwave energy, though it is possible to run the process of theinvention in a plasma apparatus without energizing the etchant to form aplasma.

Taking as an example BrCl3, a molecule of BrCl3 could hypothetically beplaced next to a silicon molecule bound to other silicon molecules incrystalline silicon, polysilicon or in an amorphous silicon matrix. Thebond energies of the Cl atoms to the Br atoms are sufficiently weak, andthe bond energy of the silicon atom to other silicon atoms issufficiently weak, and the attraction forces between Si and Cl aresufficiently strong, that without a physical bombardment of the BrCl3 onthe silicon, Cl will disassociate from Br and bond to Si to form variousproducts such as SiCl, SiCl2, SiCl3, SiCl4, etc., which etch productsare a gas a room temperature and dissipate from the silicon surface,thus removing sacrificial silicon material. The same process occurs withXeF2, BrF3 and the other vapor phase spontaneous chemical etchants.

Such chemical etching and apparatus for performing such chemical etchingare disclosed in U.S. patent application Ser. No. 09/427,841 to Patel etal. filed Oct. 26, 1999, in U.S. patent application Ser. No. 09/649,569to Patel at al. filed Aug. 28, 2000, mentioned previously, and in U.S.Patent Application 60/293,092 to Patel et al. filed May 22, 2001incorporated herein by reference. Preferred etchants for the etch aregas phase fluoride etchants that, except for the optional application oftemperature, are not energized. Examples include gaseous acid etchants(HF, HCl, HI etc.), noble gas halides such as XeF2, XeF6, KrF2, KrF4 andKrF6, and interhalogens such as IF5, BrCl3, BrF3, IF7 and ClF3. It isalso possible to use fluorine gas, though handling of fluorine gas makesthis a less desirable option. The etch may comprise additional gascomponents such as N2 or an inert gas (Ar, Xe, He, etc.). In the etchingprocess, except for optional heating, the gas is not energized andchemically etches the sacrificial material isotropically. In this way,the sacrificial material is removed and the micromechanical structure isreleased. In one aspect of such an embodiment, BrF3 or XeF2 are providedin a chamber with diluent (e.g. N2 and He). An initial plasma etch,preferably in a separate etching apparatus, can be performed prior toetching as set forth above. This sequential etch is set forth further inU.S. Patent Application 60/293,092 to Patel et al. filed May 22, 2001,the subject matter of which is incorporated herein by reference.

While the source chamber 11 can be a single chamber, the arrangementshown in FIG. 4 is an optional one in which the source chamber isactually a pair of chambers 11 a and 11 b arranged in series. The firstof these chambers 11 a contains the source material primarily in itscondensed form, i.e., either as crystals to be sublimated or liquid tobe vaporized. The second chamber 11 b receives the source material gasevolved by sublimation from the crystals or by vaporization from theliquid in the first chamber 11 a. To maintain these phases, the twochambers 11 a and 11 b will preferably be maintained at differenttemperatures (preferably at least 5 degrees C. difference), the former11 a at the lower temperature to keep the material in its condensed form(solid crystals or liquid) and the latter 11 b at the higher temperature(and possibly a higher pressure as well) to keep the material in thevapor form (and to avoid the problem of condensation) at a pressuresufficiently high to allow it to be supplied to the succeeding chambersat the pressures needed in those chambers. In one embodiment, the twochambers are held at temperatures above room temperature, with chamber11 b held at a temperature from 2 to 24 degrees C. (preferably around 5to 10 degrees C.) higher than the temperature of chamber 11 a. Such atwo-chamber embodiment could likewise be utilized in a system such asthat illustrated in FIG. 1. Chambers 11 a and 11 b could also bearranged in parallel. Also shown in FIG. 4 are the expansion chamber 12,the etching chamber 15, and pumps 18 and 88.

The terms “sample” and “microstructure” are used herein to denote thearticle from which a material is sought to be removed or to which amaterial is to be added, whether the material forms a layer among aplurality of layers, layers among a plurality of layers or a regionadjacent to other regions in the same plane. The “sample” may thus be asingle mirror element and its associated layers of other materials, atest pattern, a die, a device, a wafer, a portion of a wafer, or anyarticle from which a portion is to be removed or added. The invention isparticularly suitable for processes where the size of the portion ofmaterial that is being added or removed is less than 5 mm in at leastone of its dimensions, preferably less than 1 mm, more preferably lessthan 500 μm, and most preferably less than 100 μm. The invention is alsowell suited to adding or removing material (e.g., in the form of holesor layers) in a submicron environment (e.g. in the range of 10 nm toless than 1000 nm) (as sometimes needed, for example, in MEMS andMOEMS).

In the system depicted in the drawings, as well as other systems withinthe scope of this invention, a single charge of etchant can be placed inthe source chamber or the expansion chamber, then released (with orwithout diluents) into the recirculation loop. Additional etchant can beintroduced to replenish the circulating stream as the amount of etchantin the recirculation loop decreases over time. Other variations of theprocess will be apparent to those skilled in the art.

When the material to be removed by etching is silicon, certain etchingprocesses supply the etchant gas as a mixture of gases of which onecomponent is the etchant gas itself (or a mixture of etchant gases)while other components are non-etchant diluents. Whether the gasconsists entirely of etchant gas(es) or contains non-etchant componentsas well, the rate of the etch reaction may vary with the partialpressure of the etchant gas. The partial pressure may vary, but in mostapplications, particularly those in which silicon is being etched, bestresults will be obtained with the etchant gas at a partial pressure ofat least about 0.1 mbar (0.075 torr), preferably at least 1 Torr, morepreferably within the range of from about 1 to 760 Torr, and mostpreferably from about 50 to 600 Torr. These pressure ranges areparticularly applicable to xenon difluoride etching.

In certain processes, non-etchant gas additive(s) are included toincrease the selectivity of the silicon etch. Preferred additives forthis purpose are non-halogen-containing gases. A single such additive ora mixture of such additives can be used. In preferred embodiments ofthis invention, the additives are those whose molar-averaged formulaweight (expressed in daltons or grams per mole) is less than the formulaweight of molecular nitrogen, preferably about 25 or less, still morepreferably within the range of from about 4 to about 25, still morepreferably within the range of from about 4 to about 20, and mostpreferably within the range of from about 4 to about 10. If a singleadditive species is used, the “molar-averaged formula weight” is theactual formula weight of that species, whereas if two or more additivespecies are used in the same gas mixture, the molar-averaged formulaweight is the average of the formula weights of each species in themixture (exclusive of the noble gas fluoride) calculated on the basis ofthe relative molar amounts (approximately equal to the partialpressures) of each species. In terms of thermal conductivity, preferredadditives are those whose thermal conductivity at 300 K (26.9° C.) andatmospheric pressure ranges from about 10 mW/(m K) (i.e., milliwatts permeter per degree Kelvin) to about 200 mW/(m K), and most preferably fromabout 140 mW/(m K) to about 190 mW/(m K). In general, the higher thethermal conductivity of the additive, the greater the improvement inselectivity. Examples of additives suitable for use in this inventionare nitrogen (N2, thermal conductivity at 300 K: 26 mW/(m K)), argon(Ar, thermal conductivity at 300 K: 18 mW/(m K)), helium (He, thermalconductivity at 300 K: 160 mW/(m K)), neon (Ne, thermal conductivity at300 K: 50 mW/(m K)), and mixtures of two or more of these gases.Preferred additive gases are helium, neon, mixtures of helium and neon,or mixtures of one or both of these with one or more non-etchant gasesof higher formula weight such as nitrogen and argon. Particularlypreferred additives are helium and mixtures of helium with eithernitrogen or argon.

The degree of selectivity improvement may vary with molar ratio of theadditive to the etchant gas. Here again, the molar ratio isapproximately equal to the ratio of the partial pressures, and in thiscase the term “molar ratio” denotes the ratio of the total number ofmoles of the additive gas (which may be more than one species) dividedby the total number of moles of the etchant gas (which may also be morethan one species). In most cases, best results will be obtained with amolar ratio of additive to etchant that is less than about 500:1,preferably within the range of from about 1:1 to about 500:1, preferablyfrom about 10:1 to about 200:1, and most preferably from about 20:1 toabout 150:1. In one example, the ratio is set at 125:1.

The temperature at which the etch process is conducted can likewisevary, although the partial pressure of the etchant gas will vary withtemperature. The optimal temperature may depend on the choice of etchantgas, gaseous additive or both. In general, and particularly forprocedures using xenon difluoride as the etchant gas, suitabletemperatures will range from about −60° C. to about 120° C., preferablyfrom about −20° C. to about 80° C., and most preferably from about 0° C.to about 60° C. For xenon difluoride, the silicon etch rate is inverselyproportional to the temperature over the range of −230° C. to 60° C. Theimprovement in selectivity can thus be further increased by lowering theetch process temperature.

The flow parameters will be selected such that the time during which thesample will be exposed to the etchant gas will be sufficient to removeall or substantially all of the silicon. By “silicon” it is meant anytype of silicon, including amorphous silicon, single crystal silicon andpolysilicon, which silicon can have up to 40at % or more (typically from5 to 25 at %) hydrogen depending upon the deposition technique, as wellas impurities that can result from the target or atmosphere. Theexpression “substantially all of the silicon” is used herein to denoteany amount sufficient to permit the finished product to functionessentially as effectively as if all of the silicon had been removed. Anexample of the structures to which this invention will be applied isthat depicted in U.S. Pat. No. 5,835,256, in which a silicon nitridelayer is deposited over a polysilicon layer, and the silicon nitridelayer is patterned to leave vias that define the lateral edges of themirror elements. Access to the sacrificial polysilicon layer is throughthe vias, and the etching process removes the polysilicon below the viasby etching in the vertical direction (i.e., normal to the planes of thelayers) while also removing the polysilicon underneath the siliconnitride by etching in the lateral direction (parallel to the planes ofthe layers). The process is also effective for etching silicon relativeto multiple non-silicon layers. Also, it should be noted that thesilicon can contain impurities, and in particular may contain a largeamount of hydrogen (e.g. up to 25 at % or more) depending upon thedeposition method used.

The process design shown in FIG. 2 is but one of many designs in whichendpoint detection in accordance with the present invention can beachieved. The FIG. 2 design itself can be used with many differentcombinations and sequences of valve openings and closings. One suchsequence is as follows:

-   -   1. Solid or liquid etchant material (such as XeF2) is placed in        the source chamber(s) 11.    -   2. A sample 14 is placed in the etch chamber 15.    -   3. The expansion chamber 12 and the etch chamber 15 are each        evacuated.    -   4. The expansion chamber 12 and the etch chamber 15 are        preconditioned by a) flooding one or both of the chambers with        an inert gas (such as N2, for example), b) implementing a        temperature ramp (e.g. consisting of raising the temperature of        one or both of the chambers for fixed time followed by cooling        the temperature of one or both chambers after step 5 and        finishing with raising the temperature of one or both chambers        after step 15), or c) both flooding and implementing temperature        ramp. The sample temperature can be ramped to match or differ        from than the chamber temperature ramp.    -   5. Both the expansion chamber 12 and the etch chamber 15 are        then evacuated.    -   6. The expansion chamber 12 and the etch chamber 15 are then        filled with one or more diluents from the individual gas sources        19, 20.    -   7. The expansion chamber 12 is then evacuated.    -   8. The expansion chamber 12 is then filled with XeF2 gas from        the source chamber(s) 11 (generated by sublimation from the XeF2        crystals in the source chamber).    -   9. XeF2 gas is then pumped out of the expansion chamber 12 by        the vacuum pump 23 to lower the XeF2 gas pressure in the        expansion chamber to the desired XeF2 process pressure to be        used for etching the sample.    -   10. One or more diluent gases from the gas sources 19, 20 are        then added to the expansion chamber 12.    -   11. All valves are then closed except the manual needle valves.    -   12. The recirculation pump 18 is then activated to start a flow        of diluent gas through the etch chamber 15. Also valves 3 and 5        are opened to allow part of the recirculating gas to flow        through the gas analyzer.    -   13. The shutoff valves 26, 27 on the XeF2 recirculation loop are        then opened to cause XeF2 gas to enter the recirculation loop        36.    -   14. Recirculation of the XeF2 gas through the etch chamber is        continued until an endpoint to the etch is determined via the        gas analyzer.    -   15. Both the expansion chamber 12 and the etch chamber 15 are        then evacuated.    -   16. The expansion chamber 12 and the etch chamber 15 are        post-conditioned by a) flooding one or both of the chambers with        an inert gas, b) increasing the temperature of one or both of        the chambers, c) pumping out one or both of the chambers, or d)        following a time ordered sequence of one or more of        flooding/heating/evacuating.    -   17. The finished sample is then removed from the etch chamber.

This procedure can be varied without detriment to the product quality.Steps 12 and 13, for example, can be performed in reverse order. Othervariations will be apparent to those skilled in the art.

EXAMPLE

For etching a 6″ glass substrate with MEMS devices, typical apparatusand process parameters include: double source chamber design with 11 aat 28 C, 11 b at 31 C and intermediate connector piece at 35 C.Expansion chamber 12 and etch chamber 15 at 23 C. In step 6 above, bothchambers 12 and 15 are filled with a mixture of 45 T Nitrogen (N2) and450 T Helium (He); total gas pressure is 495 T. In step 8, the chamber12 is filled with XeF2 gas above 4.2 T. In step 9, the XeF2 gas inchamber 12 is reduced to 4 T for use in the process. In step 10, chamber12 receives 47 T of Nitrogen (N2) and 470 T of Helium (He); total gaspressure in chamber 12 at the end of step 10 is 521 T.

Endpoint Detection:

As can be seen in Toda R., Minami K., and Esashi M., “Thin Beam BulkMicromachining Based on RIE and Xenon Difluoride Silicon Etching”,Transducers '97, IEEE, pp. 671-3, Fourier Transform spectroscopy is usedto monitor the etching of silicon by xenon difluoride. The process isrun in pulse mode where the etchant gas enters the etching chamber atthe beginning of the etch, and the etching chamber is evacuated only atthe end of the etch. There is a slow build up of SiF4 in the chamberwhich gradually forms a plateau as the etch nears completion. With suchan arrangement it is very difficult to determine where along the plateauis the proper endpoint. As stated in the reference, it is consideredthat the reaction between XeF2 and silicon is mostly finished within 30seconds after the SiF4 absorption peak is nearly saturated. Attemptingto pinpoint an endpoint on the plateau of the curve of SiF4 is more of aguess than an actual calculated endpoint determination.

Another XeF2 etching method is a flow through system where an unimpededgas flows out of the etching chamber at substantially the same rate asetchant flows into the etching chamber. Such a system is disclosed in EP0878824 to Surface Technology Systems. If a gas analyzer were to beplaced at or downstream from the etching chamber for analyzing etchingproducts from the etching reaction, due to a lack of impeding the gasflow out of the etching chamber in accordance with the invention, onlynoise would be detected by such a hypothetical arrangement (see FIG. 9).

In accordance with the present invention, whether the etch system is aflow through continuous (or at least partially continuous) system, or arecirculation etch system, a gas analyzer is provided that is capable ofaccurately detecting an end point of the etching reaction. Whether thegas flow is recirculated or vented to ambient, it is desirable that thegas flow out of the etching chamber in a vented to ambient system, orthe gas flow out of a recirculation loop in a recirculation system, beimpeded to a degree so as to allow for a build up of etching reactionproduct in the etching chamber or recirculation loop (that includes theetching chamber). The impedance can be any impedance as long as it isgreater than 0 (as in the flow through system mentioned above) and lessthan infinite (as in the pulse system mentioned above). The flow can becontinuous or partially continuous (stop-start), though a pure pulsemode—filling the etching chamber with etchant and venting the etchingchamber only after the etch process is complete—is not desirable fordetecting endpoint in accordance with the invention).

Taking FIG. 1 as an example, after an etchant and diluent are mixed inthe expansion chamber, they are passed into etching chamber 14 byopening valve 15. By running pump 21 and opening valves 3 and 5, theetchant and diluent can be passed through the etching chamber during theetch process. The movement of etchant/diluent through the etchingchamber can be continuous or stop-start as long as gas from the etchingchamber is passed out of the etching chamber throughout the etch.Whether continuous or semi-continuous, the average impedance willpreferably not be infinite (a closed-off chamber during the etch) andwill be greater than 0 (when there is no build up of etching products inthe etch chamber).

Or, referring to FIG. 2, while etchant and etching products circulatefrom etching chamber 15 via recirculation line 36 and pump 18 backthrough the etching chamber, an impedance is created from thisrecirculation loop so that gas does not freely flow out of therecirculation loop, though there is a small amount of gas that flows outof the recirculation loop and into the etching chamber 12 during theetch process. In this way gas flow is impeded (less than an infiniteimpedance but greater than a 0 impedance) so that etching products willinitially build up in the recirculation line, but then decrease once thematerial being etched has been removed and the end point of the etchingreaction has been reached. The gas analyzer 1 will bleed off a verysmall amount of gas from the recirculation loop and allow for monitoringof the etching products.

The gas analyzer can be any suitable analyzer that is capable ofdetecting etch products such as gaseous SiFx molecules in a gas stream.Residual Gas Analyzers (RGA's) are available from AMETEK, AngloScientific, Ferran Scientific, Hiden Analytical, VG Gas Analysis Systemsand Stanford Research Systems. Depending upon the etch product, many gasanalysis systems could be used, including UV and visible spectrometers,Raman Spectrometers, NMR Spectrometers, Mass Spectrometers, Infrared andFourier Transform Infrared, or Atomic Spectrometers.

When gas outflow is impeded from the etching chamber as above, gascomponents monitored in a gas analyzer, particularly gaseous etchingproducts, increase and then decrease. As can be seen in FIG. 5, etchingproducts SiF3, SiF and SiF4 increase in amount (ion current in aresidual gas analyzer—RGA) up to a point around 2000 seconds, which isthe end point of the reaction. After 2000 seconds, the etching productamounts that are detected in the RGA decrease. The increase in theinitial curve is not found in a flow through system (as can be seen inFIG. 9) and the decrease at the end point is not found in pulse systems(as can be seen in FIG. 3 of the R. Toda reference mentioned above).

Taking SiF3 as an example, as can be seen in FIG. 6A, the data from theRGA forms a rising then falling curve as also illustrated in FIG. 5. Ifthese data are back averaged (e.g. with the previous 40 data points, asmoother curve results as shown in FIG. 6B. Because the average is anaverage with previously acquired data, this averaging can take place inreal time. The new averaged data of FIG. 6B can be used to take aderivative (the rate of change of the etching product), which is thedata shown in FIG. 7A. This data can also be back averaged (over 40 datapoints) to result in the curve shown in FIG. 7B. It is also possible tofurther process the curves of FIG. 6B and/or FIG. 7B with additionalcurve smoothing techniques as known in the art.

An accurate endpoint can be determined visually by a system operatormonitoring the curves of one or more etching products on a computermonitor or print-out, or preferably, the end point is automaticallydetermined based on the data from the gas analyzer. In a preferredembodiment, the end point is flagged (audio signal or visual alert). Theendpoint can be determined in a number of ways. As can be seen in FIG.6B, the RGA output increases and then decreases at a time around 2000sec (2000 sec is arbitrary and depends upon the amount of sacrificialmaterial being etched, the etchant concentration, process temperatureand pressure, etc.). A software program can be used to look for a peakvalue from the gas analyzer (corresponding roughly to the datum at time2000 sec.) or to look for a decrease (or average decrease overtime)—also taking place at around 2000 sec. in the example in FIG. 6B.In one method of the invention, the endpoint is detected after thesignal from the gas analyzer decreases for ¾ of all data points in a 25to 40 point range. In another way of determining the end point, theback-averaged data of FIG. 6B is again averaged over, e.g. 10 datapoints or more, consecutively along the curve, and when the average ofany group of 10 (or more) data points is lower than the previous 10point average, the end point is flagged.

As seen in FIGS. 7A and 7B, the derivative of the data in FIG. 6B can betaken (FIG. 7A) and then back averaged (FIG. 7B). Because graph 7Bindicates the rate of change of the data of the gas analyzer, similar tothe discussion above with respect to FIG. 6B, when the rate of changepasses across point 0 (again at time 2000 sec. in FIG. 7B) thisindicates that the rate of change of the detected etch product is nolonger increasing and is, in fact decreasing. Crossing from positive tonegative values in FIG. 7B can be monitored and flagged as the end pointof the etching reaction.

At the determined end point, if the method is being run in real time, ina preferred embodiment, the etch process is stopped—the bleeding ofetchant into the expansion chamber (or etching chamber if there is noexpansion chamber) is stopped, and any etchant and etch products arevented out of the etching chamber with an inert gas (e.g. N2, Ne or Ar).It is also possible, upon determination of the end point as above, toallow the etching reaction to proceed for a predetermined period of timeT (e.g. 20 to 100 seconds), in order to allow for slight over-etching inthe etch process. The stopping of the etch process upon end pointdetermination can be made manually or automatically.

Sacrificial silicon layers that can be removed using the apparatus andmethod of this invention may be layers of crystalline silicon, amorphoussilicon, partially crystalline silicon, crystalline silicon of multiplecrystal sizes, polysilicon in general, and silicon doped with suchdopants as arsenic, phosphorus or boron. Amorphous silicon andpolysilicon are of particular interest, although the relativecrystalline vs. amorphous character of polysilicon will varyconsiderably with the deposition conditions, the presence or absence ofdopants and impurities, and the degree of annealing. Preferably thesilicon sacrificial material is removed at a relatively slowrate—preferably less than ⅓ um/min.

Silicon can be preferentially removed relative to non-silicon materialsby the method and apparatus of this invention. The term “non-siliconmaterial” denotes any material that contains neither amorphous norcrystalline silicon in any of the forms described in the precedingparagraph. Non-silicon materials thus include silicon-containingcompounds in which elemental silicon is bonded to another element, aswell as non-silicon elements and compounds of non-silicon elements.Examples of such non-silicon materials are titanium, gold, tungsten,aluminum, and compounds of these metals, as well as silicon carbide,silicon nitride, photoresists, polyimides, and silicon oxides. Siliconnitride and silicon oxide are of particular interest in view of theiruse in the structures disclosed in U.S. Pat. No. 5,835,256. Two or moredifferent non-silicon materials may be present in a single structure,and selectivity of the silicon etch relative to all such non-siliconmaterials will be improved.

When the present invention is applied to the mirror structures disclosedin U.S. Pat. No. 5,835,256, to remove silicon layers from thosestructures, the thickness and lateral dimensions of the layers may vary.The silicon portion will generally however be a layer having a thicknessof from about 200 nm to about 5,000 nm, preferably from about 250 nm toabout 3,000 nm, and most preferably from about 300 nm to about 1,000 nm.Similarly, the non-silicon portion will generally be a layer with athickness of from about 10 nm to about 500 nm, preferably from about 20nm to about 200 nm, and most preferably from about 30 nm to about 200nm. The lateral distance that the etching process must extend under thetypical silicon nitride mirror element in the structures of U.S. Pat.No. 5,835,256 in order to remove all of the underlying polysilicon (thisdistance typically being half the shortest lateral dimension of themirror when the etching front travels inward from opposing edges) mayrange from a submicron distance to about 500 microns, preferably fromabout 3 microns to about 30 microns, and most preferably from about 5microns to about 15 microns.

While much of the foregoing description is directed to applications ofthe present invention to etching processes, the invention, andparticularly its recirculation aspect, is applicable in general toprocesses for adding or removing layers within a device, particularlylayers that have microscopic dimensions. Examples of such processes arepassivation and etching of semiconductor devices and MEMS/MOEMS devices,lithography (screen printing, for example), thin-film deposition(chemical vapor deposition e.g. application of self-assembled monolayersand spluttering, for example), electroplating (blanket andtemplate-delimited electroplating of metals, for example), and directeddeposition (as used in electroplating, stereolithography, laser-drivenchemical vapor deposition, screen printing, and transfer printing, forexample). Further examples are plasma etching, reaction-ion enhancedetching, deep reactive ion etching, wet chemical etching, electrondischarge machining, bonding (such as fusion bonding, anodic bonding,and the application of adhesives), surface modification (such as wetchemical modification and plasma modification), and annealing (such asthermal or laser annealing). The process gases in each case will bereadily apparent to those skilled in the respective arts. The presentinvention is also useful in processes where at least one of thecomponents of the process gas is corrosive to metal in the presence ofwater vapor. Corrosive components can be used for adding or removingmaterial in a microscopic device without picking up impurities that willlower the product yield and that might damage the pump used in therecirculation loop.

Further variations within the scope of the present invention are asfollows. The recirculation loop 36 of FIG. 2 can be expanded to includethe source chamber(s) 11. A valve arrangement can be incorporated intothe design that allows the operator to choose between placing the sourcechamber within the recirculation loop and isolating the source chamberfrom the recirculation loop. Similarly, diluent gas can be added to therecirculation loop simultaneously with the process gas, and anappropriate valve arrangement can be incorporated that would provide theoperator with the option to do so. Appropriate valve arrangements canalso provide the option of extending the recirculation loop 36 throughthe etch chamber 15 only or through both the etch chamber 15 and theexpansion chamber 12.

As also noted above, a filter 39 can be placed in the recirculation loop36 to filter out at least one of the byproducts or effluents produced bythe reactions occurring in the etch chamber 15, though preferably notthe etching product that is monitored for end point detection. Thisimprovement may be applied to an etching or deposition process with orwithout energetically enhancing the process gas. A selective filter thatallows the process gas to pass would be preferred. Of course, the filtercan be a basic particulate filter as well. Again, these are onlyexamples. Other variations and modifications will be readily apparent tothose skilled in the art. For example, the end point calculations cantake into account not only the data from the gas analyzer, but alsoadditional data if collected, such as data from previously run samples,change in sample weight, optical monitoring of the samples, etc. Use ofneural networks for endpoint detection are disclosed in, for example,Liamanond, S., Si, J., Yean-Ling Tseng, “Production data based optimaletch time control design for a reactive ion etching”, IEEE Trans. onSemiconductor Manufact., 2/99, vol 12, no. 1, p. 139-47, where neuralnetworks are used to model the functional relationship between an endpoint detection signal from an RIE process, as well as various in situmeasurements, and the resulting film thickness remaining.

The end point detection of the present invention can be achieved with awide variety of etch rates, though in a preferred embodiment an etchrate is selected that is slower than in the prior art. In one embodimentof the invention, the etch rate is less than 30 um/hr, and preferablyless than 25 um/hr. Slower etch rates can achieve better selectivity inthe present invention, and etch rates as low as 10 um/hr or less, oreven 7.2 um/hr or less can be performed for even greater improvements inselectivity. Though total process time is impacted, etch rates as low asabout 3 um/hr or less, 2 um/hr or less, or even 1 um/hr or less arewithin the scope of the invention. Of course within all ranges above,the etch rate is greater than 0.

Reducing the etch rate can be achieved by altering one or more of theetch parameters known to effect etch rate (e.g. etchant concentration,pressure, temperature, etc.). It is not as important which parameter(s)is used to achieve the low etch rate as long as the etch depth per timeis within the low ranges as set forth herein. Selectivity, dependingupon the etch rate, can be 500:1 (relative to a “non silicon” material,such as a silicon compound—e.g. silicon nitride or silicon oxide),1000:1, 2000:1 or even 10,000:1 or higher depending upon the etch rateand the non-silicon material.

The selectivity of the etch can be further improved by use of diluentswith the gas phase chemical etchant. The etch selectivity is increasedby using as the etching medium a gas mixture containing the etchantgas(es) and one or more of certain additional but non-etchant gaseouscomponents. While the inclusion of non-etchant gaseous additives causesprolongation of the etch time, those additives whose molar-averagedformula weight is below that of nitrogen gas prolong the etch time to amuch lesser extent than do those whose molar-averaged formula weight isequal to or greater than that of molecular nitrogen, while stillachieving the same improvement in selectivity. The improvement inselectivity is achievable independently of the partial pressure of theetchant gas in the gas mixture. Likewise, the limitation on the increasein etch time when the averaged formula weight of the additive gas isless than that of molecular nitrogen is achievable independently of thepartial pressure of the etchant gas in the gas mixture. Both theincrease in selectivity and the limitation on the etch time aresufficiently great that significant improvements in uniformity, yield,and product reliability are achieved.

These discoveries offer numerous advantages, for example in overetching,i.e., etching purposely done to a degree beyond that which is strictlyrequired for removal of the sacrificial silicon. Since the highselectivity allows one to overetch without introducing nonuniformityacross the mirror array, this invention permits the use of overetchingas a convenient and effective means of assuring complete removal of thesacrificial silicon while still preserving the integrity of the mirrorelements. The invention thus eases the requirement for precise end pointdetection, i.e., precise detection of the point at which the last of thesacrificial silicon is removed. Another advantage stems from thedilution effect of the additive gas. Dilution improves the circulationof the gaseous mixture through the system by adding to the mass thatflows through the recirculation system or agitator when such pieces ofequipment are present. Also, the presence of the additive gas helpsreduce high local concentrations of the etchant at the sample surface.Each of these factors improves microstructure uniformity and yield.

This aspect of the invention is of particular interest in etchingprocesses that are not performed in a plasma environment, i.e., etchingprocesses performed without the use of externally imposed energy such asultraviolet light or high frequency electromagnetic energy to excite thegases into a plasma state. The invention is also of particular interestin isotropic etching processes, notably those in which the silicon andthe non-silicon portions (as defined below) of the microstructure areoverlapping layers, coextensive or otherwise, or nonoverlapping layers,sharing a common boundary or separated but still simultaneously exposedto the etchant gas. The invention is particularly useful in structuresin which the silicon is a layer positioned underneath a layer of thenon-silicon material such that removal of the silicon by etchingrequires lateral access through vias in the non-silicon layer. Theinvention is also of particular interest in the manufacture ofreflective spatial light modulators of the type described in U.S. Pat.No. 5,835,256, in which the mirror elements are formed of siliconnitride or silicon dioxide and the underlying sacrificial layer servingas the support to be removed by etching is polysilicon.

As mentioned previously, etching processes addressed by this inventionare those in which the etchant is one or more gaseous noble gasfluorides, one or more gaseous halogen fluorides, or combinations ofgaseous noble gas fluorides and halogen fluorides. The noble gases arehelium, neon, argon, krypton, xenon and radon, and among these thepreferred fluorides are fluorides of krypton and xenon, with xenonfluorides the most preferred. Common fluorides of these elements arekrypton difluoride, xenon difluoride, xenon tetrafluoride, and xenonhexafluoride. The most commonly used noble gas fluoride in silicon etchprocedures is xenon difluoride. Halogen fluorides include brominefluoride, bromine trifluoride, bromine pentafluoride, chlorine fluoride,chlorine trifluoride, chlorine pentafluoride, iodine pentafluoride andiodine heptafluoride. Preferred among these are bromine trifluoride,bromine trichloride, and iodine pentafluoride, with bromine trifluorideand chlorine trifluoride the more preferred. Combinations of brominetrifluoride and xenon difluoride are also of interest.

The gas mixture is preferably contacted with the sample at a pressurebelow atmospheric pressure. The term “sample” is used herein to denotethe article from which the sacrificial silicon is sought to be removedin a selective manner relative to other materials which may reside inseparate layers or regions of the article. The “sample” may thus be asingle mirror element and its associated layers of other materials, atest pattern, a die, a device, a wafer, a portion of a wafer, or anyarticle containing sacrificial silicon. While the rate of the etchingreaction may vary with the partial pressure of the etchant gas, thepartial pressure is generally not critical to the invention and mayvary. In most applications, best results will be obtained with theetchant gas at a partial pressure of at least about 0.1 mbar (0.075torr), preferably at least about 0.3 mbar (0.225 torr), more preferablywithin the range of from about 0.3 mbar (0.225 torr) to about 30 mbar(22.5 torr), and most preferably from about 1 mbar (0.75 torr) to about15 mbar (11.25 torr). These pressure ranges are particularly applicableto xenon difluoride.

The gaseous additive that is included in the gas mixture to increase theselectivity of the silicon etch is a gas that is not itself active as anetching agent, and preferably a non-halogen-containing gas. The additivemay be a single species or a mixture of species. In preferredembodiments of this invention, the additives are those whosemolar-averaged formula weight (expressed in daltons or grams per mole)is less than the formula weight of molecular nitrogen, preferably about25 or less, still more preferably within the range of from about 4 toabout 25, still more preferably within the range of from about 4 toabout 20, and most preferably within the range of from about 4 to about10. If a single additive species is used, the “molar-averaged formulaweight” is the actual formula weight of that species, whereas if two ormore additive species are used in the same gas mixture, themolar-averaged formula weight is the average of the formula weights ofeach species in the mixture (exclusive of the noble gas fluoride)calculated on the basis of the relative molar amounts (approximatelyequal to the partial pressures) of each species. In terms of thermalconductivity, preferred additives are those whose thermal conductivityat 300 K (26.9° C.) and atmospheric pressure ranges from about 10 mW/(mK) (i.e., milliwatts per meter per degree Kelvin) to about 200 mW/(m K),and most preferably from about 140 mW/(m K) to about 190 mW/(m K). Ingeneral, the higher the thermal conductivity of the additive, thegreater the improvement in selectivity. Examples of additives suitablefor use in this invention are nitrogen (N₂, formula weight: 28; thermalconductivity at 300 K: 26 mW/(m K)), argon (Ar, formula weight: 40;thermal conductivity at 300 K: 18 mW/(m K)), helium (He, formula weight:4; thermal conductivity at 300 K: 160 mW/(m K)), neon (Ne, formulaweight: 20; thermal conductivity at 300 K: 50 mW/(m K)), and mixtures oftwo or more of these gases. For embodiments in which the molar-averagedformula weight is below that of molecular nitrogen, the preferredadditive gas is helium, neon, mixtures of helium and neon, or mixturesof one or both with one or more of higher formula weight non-etchantgases such as nitrogen and argon. Particularly preferred additives arehelium and mixtures of helium with either nitrogen or argon.

The degree of selectivity improvement may vary with molar ratio of theadditive to the etchant gas, but this ratio is generally not critical tothe utility of this invention. Here again, the molar ratio isapproximately equal to the ratio of the partial pressures, and in thiscase the term “molar ratio” denotes the ratio of the total number ofmoles of the additive gas (which may be more than one species) dividedby the total number of moles of the etchant gas (which may also be morethan one species). In most cases, best results will be obtained with amolar ratio of additive to etchant that is less than about 500:1,preferably within the range of from about 1:1 to about 500:1, preferablyfrom about 10:1 to about 200:1, and most preferably from about 20:1 toabout 100:1.

The temperature at which the etch process is conducted is likewise notcritical to this invention. The temperature does however affect thepartial pressure of the etchant gas and the optimal temperature maydepend on the choice of etchant gas, gaseous additive or both. Ingeneral, and particularly for procedures using xenon difluoride as theetchant gas, suitable temperatures will range from about −60° C. toabout 120° C., preferably from about −20° C. to about 80° C., and mostpreferably from about 0° C. to about 60° C. For xenon difluoride, thesilicon etch rate is inversely proportional to the temperature over therange of −230° C. to 60° C. The improvement in selectivity can thus befurther increased by lowering the etch process temperature.

The duration of the exposure of the sample to the gas mixture in thepractice of this invention will be the amount of time sufficient toremove all of the silicon or substantially all, i.e., any amountsufficient to permit the microstructure to function essentially aseffectively as if all of the silicon had been removed. An advantage ofthe high selectivity achieved with this invention is that it permitsoveretching of the silicon without significant loss of the non-siliconmaterial. The time required for the etching process will vary with theamount of silicon to be removed and the dimensions and geometry of thesilicon layer, and is not critical to this invention. In most cases,best results will be achieved with an exposure time lasting from about30 seconds to about 30 minutes, preferably from about 1 minute to about10 minutes. An example of the structures to which this invention will beapplied is that depicted in U.S. Pat. No. 5,835,256, in which a siliconnitride layer is deposited over a polysilicon layer, and the siliconnitride layer is patterned to leave vias that define the lateral edgesof the mirror elements. Access to the sacrificial polysilicon layer isthrough the vias, and the etching process removes the polysilicon belowthe vias by etching in the vertical direction (i.e., normal to theplanes of the layers) while also removing the polysilicon underneath thesilicon nitride by etching in the lateral direction (parallel to theplanes of the layers).

In certain procedures within the scope of this invention, the manner andthe order in which the gases in the gas mixture are combined may have aneffect on the quality of the finished product. Variations may thus beintroduced in the order of combining the etchant gas with thenon-etchant diluent or whether this is done in portions, or, when two ormore non-etchant diluents are used, the decision to combine one diluentwith the etchant gas before adding the other diluent, or which diluentor subcombination is the first to contact the sample. Such variationsmay affect parameters of the process such as the diffusion time, thereaction rate at the surface of the sample, and the rate of removal ofreaction products from the surface.

The sacrificial silicon layers to which this invention is applicable maybe crystalline silicon, amorphous silicon, partially crystallinesilicon, crystalline silicon of multiple crystal sizes, polysilicon ingeneral, and silicon doped with such dopants as arsenic, phosphorus orboron. Polysilicon is of particular interest, although the relativecrystalline vs. amorphous character of polysilicon will varyconsiderably with the deposition conditions, the presence or absence ofdopants and impurities, and the degree of annealing.

The term “non-silicon” as used herein denotes any material that does notcontain either amorphous or crystalline silicon in any of the formsdescribed in the preceding paragraph. The term thus includessilicon-containing compounds in which elemental silicon is bonded toanother element, as well as non-silicon elements and compounds ofnon-silicon elements. Examples of such non-silicon materials aretitanium, gold, tungsten, aluminum, and compounds of these metals, aswell as silicon carbide, silicon nitride, and silicon oxides. Siliconnitride and silicon oxide are of particular interest in view of theiruse in the structures disclosed in U.S. Pat. No. 5,835,256. Two or moredifferent non-silicon materials may be present in a single structure,and selectivity of the silicon etch relative to all such non-siliconmaterials will be improved.

The thickness and lateral dimensions of the layers are also noncriticalto the improvement in selectivity achieved by this invention. In mostcases, the silicon portion will be a layer having a thickness of fromabout 200 nm to about 5,000 nm, preferably from about 250 nm to about3,000 nm, and most preferably from about 300 nm to about 1,000 nm.Similarly, in most cases the non-silicon portion will be a layer with athickness of from about 10 nm to about 500 nm, preferably from about 20nm to about 200 nm, and most preferably from about 30 nm to about 200nm. The lateral distance that the etching process must extend under thetypical silicon nitride mirror element in the structures of U.S. Pat.No. 5,835,256 in order to remove all of the underlying polysilicon (thisdistance typically being half the shortest lateral dimension of themirror when the etching front travels inward from opposing edges) mayrange from a submicron distance to about 100 microns, preferably fromabout 3 microns to about 30 microns, and most preferably from about 5microns to about 15 microns.

The sample being etched comprises a layered structure formed on a quartzplate measuring 11.3 mm×15.6 mm. The first layer was a continuouspolysilicon layer deposited directly on one side of the quartz, and thesecond layer was patterned silicon nitride deposited directly over thepolysilicon layer. The polysilicon layer measured 9.2 mm×12.3 mm inlateral dimensions and was centered on the quartz surface, therebyleaving border regions along all four sides, and had a thickness of 0.5micron. The silicon nitride layer was 249 nm (0.249 micron) in thicknessand was coextensive with the quartz plate, thereby extending over boththe underlying polysilicon layer and the border regions where nopolysilicon had been deposited. The silicon nitride layer was patternedto form an array of square mirrors measuring 12 microns on each sidewith each pair of adjacent mirrors separated by a via 0.8 micron inwidth to expose the underlying polysilicon. Measurements of thethickness of the silicon nitride layer to assess the selectivity of thepolysilicon etch were performed at four locations in the border regions,close to the four comers of the quartz plate, these locations beingspaced apart from the edge of the polysilicon layer by distances greaterthan 300 microns. This distance was chosen to assure, for purposes ofuniformity, that the measurement locations experienced no temperaturerise from the exothermic polysilicon etch reaction, since the thermallyinsulating nature of silicon nitride precluded any such temperature riseat locations beyond approximately 100 microns from the edge of thepolysilicon layer.

The time required for full removal of the polysilicon layer wasdetermined by visual observation, as indicated above. Of course, thetime needed for full removal of the sacrificial layer could also beperformed in accordance with the end point detection methods (monitoringgas reaction products) as set forth previously herein. The thickness ofthe silicon nitride at the measurement locations was determined bothbefore and after the polysilicon etching by a common industry method ofthin-film measurement using the reflectance of the film (as used in theNanoSpec Thin Film Measurement System of Nanometrics, Inc., Sunnyvale,Calif., USA, and in the Advanced Thin Film Measurement Systems ofFilmetrics, Inc., San Diego, Calif., USA). Measurements were performedon two or three samples for each experiment, and the results averaged.The results are listed in the table below, which include as the firstexperiment a control run with xenon difluoride alone and no additive.

Experimental Results Experimental Results Gas in the Gas in the Timerequired Si₃N₄ thickness loss (Initial Experiment No. of 1^(st) Gas2^(nd) Gas for removal of Thickness 249 nm during No. Samples SourceSource Polysilicon Polisilicon removal) I 3 None None  65 sec 11-13 nm II 3 N₂ N₂ 610 sec 2-3 nm III 2 Ar Ar 590 sec 2-3 nm IV 2 He He 250 sec2-3 nmWith the etching of the underlying polysilicon layer in the lateraldirection, the etching distance of the polysilicon was one-half thewidth of each mirror element, or 0.5×12 microns=6,000 nm. The results inthe table indicate that the selectivity of the etch of polysiliconrelative to silicon nitride rose from approximately 500:1 (6,000 nm: 11nm) with the xenon difluoride-only etch medium in Experiment I toapproximately 2,000:1 (6,000 nm: 3 nm) with the addition of each of theadditive gases in Experiments II, III and IV, and that the increase inetch time of the polysilicon when the additive was helium (ExperimentIV) was well under half the attendant increases when the additives werenitrogen and argon, both of which had formula weights exceeding 25.These diluents are but examples, and any diluent or combination ofdiluents can be used, though preferably as long as the etch rate iswithin the low etch rate ranges of the invention.

As can be seen from the above various etch rates can be used to removethe sacrificial layer, including an etch rate can be 27.7 um/hr (0.5 umetched in 65 sec.) such as if no diluent is used, or lower (e.g. 25 or20 um/hr or less). For example, an etch rate of 7.2 um/hr (0.5 um etchedin 250 sec.) can be achieved with a helium diluent. Even lower etchrates are achieved in an argon diluent or in a nitrogen diluent (0.5 umetched in 590 or 610 sec=3 um/hr). Other diluents and mixtures ofdiluents can be used, though it is preferred that the etch rate be 10um/hr or less, 3 um/hr or less, or even 2 um/hr or 1 um/hr or less.

In a further embodiment of the invention, a MEMS device is formed wherea sacrificial layer (or layers) is deposited on a substrate. During orafter deposition the sacrificial layer, the sacrificial material isdoped with a dopant. The doping can occur during deposition of thesacrificial material, such as feeding a dopant into the process gasduring a chemical vapor deposition of the sacrificial material. Or, thesacrificial material can first be deposited, followed by implanting thesacrificial layer with the dopant (e.g. phosphorous, arsenic, boron orother semiconductor dopant). In a preferred embodiment of the invention,the sacrificial layer is silicon (e.g. amorphous silicon,polycrystalline silicon), the material that is not to be etched is anon-silicon material, such as a metal (Al, Ti, Au, etc.) or metalcompound (e.g. a nitride of titanium, aluminum tantalum-silicon,tungsten or an oxide of aluminum, silicon, tantalum, titanium, etc. or ametal carbide), where the silicon sacrificial is doped with the dopant.The dopant can be any dopant (e.g. borane, arsine or phosphine), thoughpreferably one that improves the selectivity of the etch. Possibledopants if the sacrificial material is silicon, include PH3, P2H5, B2H5,BCl3, etc. The dopant can be implanted in accordance with standardsemiconductor manufacturing implanting methods, or mixed into theprocess gas while depositing the sacrificial material, e.g. inaccordance with such doping methods used in making solar cells. Otherdoping methods (ion transfer via thermal anneal, etc.) could also beused. The dopant can be used to dope only a top portion of thesacrificial layer, or the dopant can be made to be present throughoutthe sacrificial material. Doping can be at 10¹⁰ to 10¹⁸ ions/cm³, suchas around 10¹⁴ ions/cm³. Implantation can be performed at an energy of10 to 70 keV, preferably from 20 to 40 keV. Other implantation densitiesand energies could also be used.

In the present invention, the silicon can be polysilicon as set forthabove, or amorphous silicon deposited by LPCVD or PECVD, or sputtering,or other materials and techniques as set forth in U.S. patentapplication Ser. No. 09/617,149 to Huibers et al. filed Jul. 17, 2000,U.S. patent application Ser. No. 09/631,536 to Huibers et al. filed Aug.3, 2000, U.S. patent application Ser. No. 09/767,632 to True et al.,filed Jan. 22, 2001 and/or Ser. No. 09/637,479 to Huibers filed Aug. 11,2000, each incorporated herein by reference. Other micromechanicalstructural materials and methods can be used other than those set forthabove, such as those materials set forth in U.S. Patent Application60/293,092 to Patel et al. filed May 22, 2001, U.S. Patent Application60/254,043 to Patel et al. filed Dec. 7, 2000, U.S. patent applicationSer. No. 09/910,537 to Reid filed Jul. 20, 2001, and U.S. PatentApplication 60/300,533 to Reid filed Jun. 22, 2001, each incorporatedherein by reference.

Though the apparatus and process disclosed herein are for etching amaterial from any work piece (semiconductor device, MEMS device, deviceto be cleaned of silicon residue, etc.), in one embodiment the materialbeing removed is a sacrificial layer in a MEMS fabrication process. Aspecific example of a MEMS device that could be made in accordance withthe invention is a micromirror array such as disclosed in U.S. Pat. Nos.5,835,256 and 6,046,840 to Huibers et al. The MEMS device, of course,could be any device, including movable mirror elements for opticalswitching such as disclosed in U.S. patent application Ser. No.09/617,149 to Huibers et al. filed Jul. 17, 2000. Each of the abovepatents and applications are incorporated herein by reference.

The foregoing description and examples are offered primarily forpurposes of illustration. It will be readily apparent to those skilledin the art that numerous modifications and variations beyond thosedescribed herein can be made while still falling within the spirit andscope of the invention.

1-100. (canceled)
 101. An apparatus for etching a sample, said apparatuscomprising: (a) a source of etchant gas; (b) an etching chamber incommunication with said source of etchant gas; (c) a recirculation looppassing through said etching chamber; (d) a pump disposed within saidrecirculation loop for recirculating etchant gas along saidrecirculation loop; and (e) a gas analyzer for analyzing gas componentswithin the recirculation loop.
 102. The apparatus in accordance withclaim 101 which said source of etchant gas comprises a source chamber.103. The apparatus in accordance with claim 102 further comprising anexpansion chamber communicating with said source chamber and with a gassource for a gas other than said etchant gas, said expansion chamberarranged for mixing gas from said source chamber with gas from said gassource.
 104. The apparatus in accordance with claim 103 which saidexpansion chamber is in communication with said recirculation loop. 105.The apparatus in accordance with claim 101 further comprising a filterdisposed within said recirculation loop, said filter being one thatremoves a member selected from the group consisting of byproducts oreffluent from gases flowing through said recirculation loop, orparticulates.
 106. The apparatus in accordance with claim 101 in whichsaid pump is a dry pump.
 107. The apparatus in accordance with claim 106in which said dry pump has no wet seals and adds no gas to saidrecirculation loop.
 108. The apparatus in accordance with claim 107 inwhich said dry pump is a bellows pump.
 109. The apparatus in accordancewith claim 108 in which said bellows pump comprises a housing withbellows-type wall sections enclosing a hollow interior, and at least onepartition disposed to divide said hollow interior into a plurality ofsections.
 110. The apparatus in accordance with claim 101 in which saidpump is constructed to circulate etchant gas substantially continuouslywithin said recirculation loop.
 111. The apparatus in accordance withclaim 103 in which said pump is defined as a first pump and saidapparatus further comprises a second pump arranged to draw gases from amember selected from the group consisting of said expansion chamber,said source chamber, and said recirculation loop.
 112. The apparatus inaccordance with claim 103 further comprising gas flow spreading means insaid source chamber for diverting incoming gas.
 113. The apparatus inaccordance with claim 112 in which said gas flow spreading means is abaffle.
 114. The apparatus in accordance with claim 112 in which saidgas flow spreading means is a perforated plate.
 115. The apparatus inaccordance with claim 101, further comprising an energy source and/orelectric field source at the etching chamber for forming a plasmatherein.
 116. The apparatus in accordance with claim 102 in which saidsource of etchant gas further comprises fluoride crystals retainedwithin said source chamber.
 117. The apparatus in accordance with claim116 in which said fluoride crystals are xenon difluoride crystals. 118.The apparatus in accordance with claim 103 in which said gas source fora gas other than said etchant gas comprises a source of a gas with molaraveraged molecular weight less than or equal to that of N2.
 119. Theapparatus in accordance with claim 118 in which said gas other than saidetchant gas is a member selected from the group consisting of Ar, Ne, Heand N2.
 120. The apparatus in accordance with claim 103 in which saidgas source for a gas other than said etchant gas comprises a pluralityof gas sources, the gases from which, when mixed, yield a gaseousmixture with molar averaged molecular weight less than or equal to thatof N2.
 121. The apparatus in accordance with claim 120 in which saidplurality of gas sources are sources of two or more members selectedfrom the group consisting of Ar, Ne, He and N2.
 122. A method,comprising: a) providing a sample to be etched in a chamber; b)providing an etchant to the chamber, capable of etching the sample; c)providing no or substantially no impedance to gas exiting the etchingchamber; d) monitoring a partial pressure of an etch product; and e)repeating steps a) to d) except providing an increased impedance eachtime steps a) to d) are repeated, until an impedance is reached thatallows for determining an endpoint based on monitoring the partialpressure of the etch product.
 123. The method of claim 122, wherein animpedance is reached that results in a partial pressure that begins todecrease at or near a time that the endpoint of the etch is reached.124. The method of claim 122, wherein an endpoint of the etchcorresponds to a point where all of the material has been removed. 125.The method of claim 122, wherein the material is silicon.
 126. Anapparatus comprising: an etching chamber; a source of an etchant; a gasrecirculation loop for recirculating the etchant repeatedly through theetching chamber; and a gas analyzer within the etching chamber or withinthe gas flow line downstream of the etching chamber.
 127. The apparatusof claim 126, wherein the gas analyzer is a spectrometer.
 128. Theapparatus of claim 127, wherein the spectrometer is capable of detectinglevels of fluoride etchants or fluoride etching products.
 129. Theapparatus of claim 126, wherein the gas analyzer is a residual gasanalyzer.
 130. The apparatus of claim 126, further comprising asemiconductor or visible light transmissive wafer held on a wafer chuckin the etching chamber. 131-135. (canceled)